1. Field of the Invention
The invention relates to a boundary tracking apparatus, and more particularly, to a boundary tracking apparatus in an orthogonal frequency division multiplexing (OFDM) system and a method thereof.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a block diagram of a typical orthogonal frequency division multiplexing (OFDM) frame. According to the IEEE 802.11 a standard, the frames of the OFDM system have a short preamble in the beginning, a long preamble following afterwards, and a guard interval GI2 between the short preamble and the long preamble. There are also guard intervals GI between each data block to avoid the inter-symbol interference (ISI). The guard interval GI and GI2 can be a cyclic prefix of a data block or a cyclic prefix of the long preamble.
In conventional boundary detecting methods, a receiver detects the beginning of the long preamble and the beginning of the data blocks according to a period and the auto-correlation of the short preamble. The guard intervals GI are then removed accordingly to plan a boundary of the data blocks for fast Fourier transforming (FFT). Next, the conventional boundary detecting methods execute a fast Fourier transform on each data block according to the estimated channel responses of all the sub-carriers with different boundaries to determine a proper boundary.
However, the above-mentioned method usually mistakenly estimates the guard interval of the long preamble with a serious delay spread. Therefore, the above-mentioned methods choose a wrong timing to execute the fast Fourier transform. Furthermore, the prior boundary tracking methods are inefficient to compute for several times the FFT of numerous symbols according to every sub-carrier. That is, tracking the boundary by comparing the FFT of each symbol takes a lot of effort.